The present invention relates generally to amplifiers and more specifically to an amplifier having improved slewing characteristics.
A two-stage amplifier, as illustrated in FIG. 1, includes an input stage A1 and an output stage A2. Connected across the output stage is a compensation capacitor C.sub.C. The input stage includes two differential input transistors Q1 and Q2 and a current mirror having output transistors Q3 and Q4. The collectors of the transistors Q1 and Q2 are connected to a current source CS1. The collector currents IC of transistors Q1 and Q2, for small differential voltages less than 50 millivolts, determines the AC, noise and DC performance 15 parameters. In a typical amplifier the current available to slew the compensating capacitor C.sub.C is equal to two times the normal collector current expressed as ##EQU1##
To obtain higher slew rates, designers have two options, namely, to decrease the size of the compensation capacitor C.sub.C, or increase the current available to charge the capacitor C.sub.C. Decreasing the size of the compensation capacitor C.sub.C is undesirable because this directly affects the small signal parameters. A more desirable alternative is generally to design for small signal parameters and then add a circuit which delivers additional current to slew the compensation capacitor under large signal conditions.
One method to provide the additional current, as described in U.S. Pat. No. 4,240,040 is to provide a range extender stage in parallel with a balanced common base stage between the input and output stages each having its own current source. Another method is to provide a large signal stage in parallel with the small signal stage at the front end or input stage. The large and small signal stage each has their own current source. This method is described in co-pending U.S. Ser. No. 782,690 filed Oct, 1, 1985 to Gerald M. Cotreau.
The use of separate current sources for the small signal stage and large signal stage adversely affects the settling time. Most amplifiers use the current enhancement to increase the slew rate. The settling time specification includes the large signal slewing, and the time required for the output to settle to a specified error band around the final value. Separate stages have an AC stage in parallel with a stage which supplies large amounts of current. A problem exists with the current enhancement stage switching off and thus effecting settling time. Another problem exists with non-linearity. When a large signal is applied, the output is expected to slew to its final value. However this may not be the case. These slew enhancers schemes will display three regions of operation when a small signal pulse is applied. First will be the small signal rise time, second is the dead band where the amplifier slews without the assistance of the current enhancement, and third is the slewing response, as illustrated in FIG. 2.
Thus, it is an object of the present invention to offer a circuit which increases the output current for large signal conditions with an insignificant degradation of AC, noise and DC performance parameters of the differential pair.
Another object is to provide a linear enhancement without the adverse affects of switching.
Still another object is to provide a slew rate enhancement with minimum settling time and output response dead band.
These and other objects of the invention are attained by providing a current source capable of providing amplification and slewing currents and a large signal circuit connected to the current source for shunting varying amounts of slewing current from the amplifier inversely proportional to differential input voltage. The large signal circuit includes a current sink capable of sinking the slewing current and a linear controller connecting the current sink to the current source. The large signal circuit is responsive to the differential input voltage to turn up the linear controller and cause the current sink to draw more current from the current source for small differential inputs and to turn down the linear controller and to cause the sink to draw less current from the current source to allow the current source to provide amplification and slewing current to the amplifier for large differential inputs. The amplifier includes a differential pair of transistors and the large signal circuit also includes a pair of differential transistors connect parallel to the amplifier differential transistors to control the linear controller connecting the current source and the current sink. In this instance the linear controller is similar in operation to a biased output stage rather than being a switch-type differential voltage amplifier.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.